The invention relates to an image processor and an image processing method used for image processing such as halftoning, which is used for generating an electrophotograph by use of, e.g., a color laser printer.
Depending on the type of a print engine, limitations are generally imposed on the number of colors (CMYK or CMY) and the number of levels of gradation (e.g., two levels of gradation), which are available in a color laser printer. For this reason, difficulty is encountered in faithfully regenerating, in the form of printed matter, a natural color image having continuous multiple gradations; e.g., a photograph.
For this reason, aggregates of minute dots have hitherto been used to artificially represent multiple gradations and colors, thereby creating an optical illusion for a person viewing the printed matter. In relation to the optical illusion, the greater the number of dots provided in a certain small area, the greater the apparent density of the entire small area. Conversely, the smaller the number of dots decreases, the lower the apparent density of the small area. Similarly, print areas of respective print colors; that is, CMY or CMYK, are adjusted by utilization of the optical illusion, whereby the number of colors is artificially increased through use of limited print colors, thus regenerating colors of an original image.
In this way, a method for artificially increasing the number of colors by adjustment of print areas of respective colors over a wider region or expressing multiple gradations in the form of dot areas is called an areal gradation method. A dithering technique and an error diffusion technique, for example, have been known as embodiments of the areal gradation method.
When the areal gradation method is used for converting, e.g., a gray-scale image having multiple gradations into a black-and-white binary image, an original image is divided into blocks of predetermined shapes (halftone cells). The thus-divided pieces of images are subjected to binary-encoding in the respective blocks. However, there may arise a case where, when color planes employing blocks of the same shapes are superimposed one on another, interference, known as moiré fringes, appear between patterns.
In particular, according to the dithering method, the entire original image is divided through use of blocks of identical shape; e.g., a rectangular shape of fixed size. Further, the respective blocks have the same layout of dithering threshold levels. Therefore, when the blocks are rasterized, identical patterns are likely to appear recursively. When color planes having the same raster patterns are superimposed one on another, moiré fringes may appear. For this reason, the chance of occurrence of moiré fringes is reduced by adjusting the directions in which blocks are arranged in the respective color planes (i.e., screen angles).
By way of an example, the darkest (black (K)) plane is placed at an angle of 45°; a cyan (C) plane is placed to cross the black (K) plane at an angle of 30° (i.e., 15°); a magenta (M) plane is placed to cross the black (K) plane at an angle of 30° (75°); and a visually-inconspicuous yellow (Y) plane is placed at an angle of 0° (i.e., an intermediate position between the cyan (C) plane and the magenta (M) plane, because an angle of 75° is equal to an angle of −15°).
However, difficulty is encountered in superimposing colors of print planes at accurate angles. In particular, in the case of a so-called tandem color laser printer equipped with print engines for respective colors, the printer encounters difficulty in improving the accuracy of registration of colors (i.e., a color registration), because of its principle. For this reason, adjustment of screen angles is limited in its effectiveness in lessening moiré fringes.
According to the error diffusion technique, errors are distributed over all pixels such that accumulated errors arising between original pixels and analogous pixels become zero. Hence, in contrast with the dithering technique, the error diffusion technique has no periodicity and is less likely to generate a moiré fringe. However, minute dots (in particular, isolated dots) of an electrophotograph are unstable. Hence, use of the error diffusion technique entails problems, such as instability of colors and a failure to ensure a large number of gradation levels.